Tuesday, 11 January 2005: 11:45 AM
The Relationship between Cloud Droplet Distributions and Ambient Aerosol Populations in a Subtropical Desert Region
On first assessment, the clouds that form over the desert and mountains of the United Arab Emirates appear to be good candidates for hygroscopic seeding. The cloud droplet concentrations, especially in the summer convective clouds, are generally greater than 500 cm -3 and have liquid water contents that range from 0.2 to 0.6 g m-3. This suggests a fairly narrow droplet size distribution and agrees with the large aerosol concentrations observed during flights made during 2001 and 2002. However, analysis of aerosol composition suggests that many of the aerosols in this region are made of up an external mixture of anthroprogenic sulfates, sea salt, and desert dust which may have extrusions of sulfate in them. Normally, desert dust is hydrophobic. The inclusion of the hygroscopic sulfates make desert dust a possible giant cloud condensation nuclei (GCCN) as well as a potential ice freezing nuclei (IFN). This paper will investigate the differences between the size distribution of these cloud droplets near the cloud base when there are and are not an aircraft observed presence of dust near the top of the boundary layer and will be extended to look at the correlation between desert dust and observed ice populations above the freezing level. It will then assess these observations in light of the feasibility of using hygroscopic seeding to enhance rainfall in this region.